Sulfur is a major contaminant in raw materials used in petrochemical production, with extensive regulations in place to minimize the sulfur content of final products. In most regions, hydrotreatment results in the removal of sulfur from various liquid and gas streams in the form of hydrogen sulfide (H2S). This H2S is then further treated to recover elemental sulfur (S), typically by the Claus process.
The Claus process utilizes heat, catalysis, and oxygen (O2) to convert a portion of the recovered H2S to sulfur dioxide, as follows:2H2S+3O2→2SO2+2H2O+heat.The resulting mixture of H2S and SO2 reacts to produce elemental sulfur, as follows:2H2S+SO2→3S+2H2OThis second reaction is commonly referred to as the Claus reaction. While a large amount of sulfur can be recovered as elemental sulfur in such a process, in real world processes, reactors performing the Claus reaction produce a tail gas that includes H2S, hydrogen (H2), carbon monoxide (CO), carbon disulfide (CS2), sulfur dioxide (SO2), and carbonyl sulfide (COS). In order to meet sulfur emission targets, such Claus tail gas frequently requires additional treatment.
Conventional Hydrogenation/Formulated Amine Claus Tail Gas Treating Unit (TGTU) is capable of reducing the H2S concentration of a hydrogenated/hydrolyzed Claus Tail Gas to about 5 ppmv. However, due to high CO2 content (usually above 40% in the acid gases of coal gasification and gas plant facilities), the COS concentration in the subsequent hydrogenated/hydrolyzed. Claus tail gas will be very high, typically between 100-300 ppmv. This is due to the chemical and thermal equilibrium of the following reaction:CO2+H2SCOS+H2O
While amine-containing solutions are effective in recovering H2S, such solutions have little effect on the COS component in this hydrogenated/hydrolyzed Claus tail gas. As a result, while an amine absorber overhead gas effluent may contain only 5 ppmv or less H2S, it can still contain a substantial amount of sulfur in the form of COS. The total amount of H2S and COS in such an absorber overhead effluent, following incineration, may result in a SO2 concentration that exceeds what is allowed by local regulations. To reduce the SO2 concentration to below 100-400 mg/Nm3 (milligrams per standard cubic meters, wherein standard conditions are taken at 0° C. and 1013 millibar) can be costly and potentially creates another waste stream to be dealt with (for example, if a caustic wash technology were used as an additional sulfur removal step).
Thus, there is still a need for systems and methods that provide effective and efficient removal of COS from Claus process tail streams.